This invention relates to methods of potting filtering hollow fibre membranes into a header and to headers of potted hollow fibre membranes.
In order to filter or permeate with hollow fibre membranes, a large number of thin hollow fibres must be fixed to a header such that their outer surfaces are each completely sealed to the outside of header but their lumens are open to an inner space in the header. The inner space of the header is then connected to a source of suction or pressure to create a transmembrane pressure across the walls of the membranes.
In U.S. Pat. No. 5,639,373, the ends of an array of spaced apart fibres are submerged in a fugitive liquid, such as a wax, until the fugitive liquid solidifies around them. A fixing liquid, such as a resin, is then poured over the fugitive liquid and allowed to harden around the membranes. The fugitive liquid is then removed, for example by heating or by dissolution, leaving the lumens of the membranes open to the space formerly occupied by the fugitive liquid. In U.S. Pat. No. 6,042,677, a similar process is used but the array of fibres is held in a bed of powder which is used in place of the solidified fugitive liquid.
In U.S. Pat. No. 5,922,201, a continuous hollow fibre is made into a fabric such that adjacent lengths of the fibres are spaced apart from each other and do not have open ends. An edge of the fabric is inserted into a pot of liquid resin which is centrifuged or vibrated as it cures to encourage flow into the spaces between the fibres. After the resin is cured, the block of resin and fibre is cut to separate the fabric into individual lengths of fibres having open ends. The block of resin is then glued or attached through gaskets to the remainder of a header. The use of a centrifuge and the need to later attach the cured block of resin to the remainder of the header add to the cost and complexity of the method. Further, the ends of the fibres may be damaged when the fibres are cut while encased in resin.
In European Patent Application No. EP 0 931 582, an elastic pipe is used as a header. An aperture is cut in the pipe and a weir is built up around the aperture. Open ends of hollow fibre membranes are inserted spaced apart in a line into the aperture by first pulling the aperture open and then allowing it to close on the membranes. Liquid resin is poured over the ends of the membranes and retained in placed by the weir until it cures. Surface tension prevents the resin from flowing through the aperture in spaces between adjacent fibres but only a single layer of fibres are potted in each aperture.
The inventors have observed various difficulties with the method in U.S. Pat. No. 5,639,373. One difficulty is that the fugitive liquid wicks up the fibres to a certain wicking height. In order to secure a good bond to the outside of the fibres, the fixing liquid is applied to a depth that exceeds, by a required distance, the wicking height of the fugitive liquid. With large diameter fibres, around 2.0 mm outside diameter for example, the wicking height is about 2 to 10 mm. This requires some excess fixing liquid to be used at some increase in cost but the amount of excess fixing liquid is manageable. With smaller diameter fibres, for example about 1.0 mm outside diameter or less, the wicking height of the fugitive liquid can be 5 to 20 mm. Particularly at these wicking heights, the required excess thickness of the fixing liquid becomes significant and undesirable.
Another difficulty with the U.S. Pat. No. 5,639,373 method is that the fibres are organized into a spaced apart relationship before insertion into the fugitive liquid. This is done because the fugitive liquid, when solidified, holds the fibres in whatever relationship that exists when the fibres are placed in the fugitive liquid. Although a very deep layer of the fixing liquid might adequately separate a random arrangement of fibres fixed in the solidified fugitive liquid, pre-spacing the fibres is a preferred solution even though it adds a step to the U.S. Pat. No. 5,639,373 process.
Yet another difficulty with the U. S. 5,639,373 process is that the fugitive liquid is often difficult to work with. Solidified fugitive liquids that are dissolved with a solvent generate solvent handling and disposal concerns and limit the choice of permeate pan material to those that do not react with the solvent. Solidified fugitive liquids that are to be melted are typically made of a wax so that their melting temperature is low. Waxes, however, react with many otherwise suitable permeate pan materials and may also produce manageable but undesirable minor reactions with useful fixing resins. For these reasons, permeate pans for use with the U.S. Pat. No. 5,639,373 process are typically made of expensive fibreglass reinforced plastics.
Finally, there is sometimes a difficulty in potting headers with xe2x80x9cfibre twinning.xe2x80x9d In fibre twinning, the fixing liquid wicks up the fibres by 1 to 2 cm before it hardens and joins two (or possibly a few) fibres together for a short distance above the top of the header. Thus, one side of the base of a fibre may be attached to another fibre while the other side of the base of the fibre is not contained in solidified fixing liquid. Under intense aeration or physical handling of the membranes (typically, unintentionally during shipping or maintenance or intentionally as part of physical de-sludging) the base of the fibre may be bent towards its uncontained side. The fibre may be damaged if it rips free of the resin that bonds it to a neighbouring fibre. In particular, where composite membranes are used (such as a coated braid as described in U.S. Pat. No. 5,472,607 or a polysulfone membrane coated with PVDF), the outer layer may stick to the resin while the remainder of the fibre pulls free. Since the outer coating typically contains the smallest pores, a defect is created in the fibre. The inventors have observed fibre twinning with the method in U.S. Pat. No. 5,639,373 but believe that it is likely present in all of the prior art methods described above. The inventors expect that fibre twinning may be less of a concern with centrifuged headers, although in those cases the cost and complexity of centrifuging is itself a concern. In summary, the inventors have noticed many areas in which membrane potting technology, including the process in U.S. Pat. No. 5,639,373, may be improved.
It is an object of the present invention to improve on the prior art. This object is met by the combination of features, steps or both found in the claims. The following summary may not describe all necessary features of the invention which may reside in a sub-combination of the following features or in a combination with features described in other parts of this document.
In various aspects, the invention provides a method of potting filtering hollow fibre membranes into a header. A plurality of hollow fibre membranes are collected together and their open ends are inserted into a dense, viscous liquid, suspension or, preferably, a gel in a container. The gel has sufficient viscosity and surface tension such that it does not wick up the fibres significantly and sufficient density to remain below a fixing liquid, typically an uncured resin, to be placed above the gel. The fixing liquid surrounds each membrane and then becomes a solid sealingly connected to the outside of each membrane but not blocking the lumens of the membranes.
The membranes preferably have outside diameters of about 1 mm or less, for example between 0.5 and 0.7 mm. The plurality of hollow fibre membranes may be arranged before they are potted randomly in a bundle. If randomly arranged before potting, the packing density and membrane and gel material are preferably selected such that the gel tends to disperse the membranes and create a desired closely spaced arrangement. The fixing liquid also wets the membranes and further separates them from each other. Since the ends of the membranes are only partially constrained by the gel, the fixing liquid may surround and space the membranes even if some membranes initially touch each other in the gel. Alternatively, the membranes can be pre-arranged to be closely spaced apart before inserting their open ends into the gel, particularly if a pre-determined spacing is desired.
Preferably, the potting method is performed in the header. Header pans are prepared of a material which is substantially unreactive with the fixing liquid or the gel. This typically includes a broad range of materials of which ABS is preferred because of its low cost, durability and ease of molding or fabricating into a desired shape. Header pans are prepared with an opening to an inner space defining a permeate channel. The gel is placed in the header in the space reserved for the permeate channel. The open ends of the membranes are then inserted into the gel. The fixing liquid is placed over the gel. When the fixing liquid solidifies, it simultaneously seals the outer surfaces of the membranes and forms a plug in the opening of the header pan to complete the permeate channel. After the fixing liquid has solidified, the gel is removed. The solidified fixing liquid remains attached to the header pan in a position where the open ends of the membranes can be in fluid communication with the permeate channel. The space initially occupied by the gel becomes part of the permeate channel after the gel is removed.
Preferably, the fixing liquid is a resin which continues to cure after it has solidified and a substantial portion of the gel is permitted to flow as a gel out of the header while the resin cures. The remaining gel can be removed by dissolving it or by mechanical means such as flushing with water. Further, the gel may be and is preferably thixotropic and can be removed in part by vibrating the gel to a liquid state. A thixotropic gel can also be vibrated to assist in placing the gel evenly in the header. The gel is also soluble in a solvent that does not dissolve the solidified fixing liquid. The solvent is preferably water and the gel may also be removed in part by dissolving the gel in the solvent. The gel may also be heated to assist in placing it in the header or later removing it.
In another aspect, filtering hollow fibre membranes are potted in a header by first preparing a group of preferably closely spaced hollow fibre membranes surrounded and held together by a layer of solidified adhesive. The layer of solidified adhesive is located near the ends of the fibres but with the ends of the fibres extending beyond a first side the adhesive. A fixing liquid is placed around the membranes such that the fixing liquid extends from the periphery of the adhesive towards the ends of the membranes. The fixing liquid surrounds each membrane at least at a point between the adhesive and the open end of each membrane. The fixing liquid becomes a solid sealingly connected to the outside of each membrane but not blocking the lumens of the membranes and not contacting the membranes where they exit from a second side of the adhesive. The solidified fixing liquid is attached to a header pan in a position where the open ends of the membranes can be in fluid communication with a permeate channel in the header.
The adhesive is chosen to be water insoluble and durable in a solution of any chemicals likely to be present in a substrate to be filtered. The adhesive preferably does not wick up the membranes to any significant degree and the adhesive/fibre bond is preferably weaker than the bond between any layers of a composite fibre. A suitable adhesive is polyethylene hot melt adhesive such as a mix of ethelene vinyl acetate co-polymers when used with fibres having an outer surface of polysulfone, polypropylene or PVDF. The mix of adhesive components is preferably chosen using techniques known to those skilled in the art to be fairly soft and flexible so as to cushion the membranes where they exit the second side of the adhesive.